Epidemiology – the basics cont.

11/9 2018

Nils Fall, DVM, PhD Division of Ruminant Medicine and Veterinary Department of Clinical Sciences Swedish University of Agricultural Sciences Measures of and associations The epidemiological toolbox • Measures of disease occurrence • • • Cumulative incidence (or Incidence proportion) • Incidence rate • Measuring effects • Measures of strength of association • /risk ratio • • (Incidence rate ratio) Quantifying disease occurrence

Prevalence Incidence “prevail” “incident” How many are ill? How many fall ill? Total population Prevalence

diseased

Incidence Healthy individuals diseased Prevalence proportion = point prevalence Definition: The proportion of the population with a particular characteristic at a given point in time.

Formula: Number of existing cases at a point in time The number of people in the population at that time Aviod ”dangling numerators”!! Notes on prevalence • Time must be defined

• Values between 0 and 1 (0-100%)

• Measure of the overall at a given point in time

• A proportion without units • “prevalence proportion” Measures of occurence

Prevalens Incidence “Prevail” “incident” How many are ill? How many fall ill?

“Incidence proportion” “Incidence rate” Incidence proportion, cumulative incidence or incidence risk Definition: The number of new cases of a disease in a certain population during a specified time period

Formula: Number of new cases during follow up-period Number of persons at risk at start of follow up

Healthy animals Diseased What is the incidence proportion of disease (=)?

Person Year of follow-up

12345678910

A 

B

C 

D 

E 

F 

G

H  Notes on incidence proportion • The time period must be specified • Proportion - unitless from 0 to 1 • Only those at risk in the denominator • Approaches 1 with time (“cumulative incidence”) • Problem with drop outs during follow up! • If a very short follow-up period (e.g. outbreak) •“” (AR) What is the incidence proportion here? Person Year of follow-up

12345678910

A 

B

C 

D  moved!

E 

F  moved! G 

H  Measures of disease occurrence

Prevalence Incidence How many are ill? How many fall ill?

Incidence proportion Incidence rate Incidence rate (IR) Definition: The number of new cases per animal/person time (speed of transition from healthy to diseased)

Formula: Number of new cases during follow up The sum of the time at risk for each subject at risk

Unit: cases per animal/ person-days, -months or -years What is the incidence rate here? Person Year of follow-up

12345678910

A 

B

C 

D  moved!

E 

F  moved! G 

H  Notes on incidence rates • Unit is “cases per animal/ person-time”, range: 0 to ∞ • (rate ≠ proportion)

• Useful in unstable populations (loss to follow-up)

• Useful when the follow-up period is long

• Only “at-risk animal/person time” in the denominator

• Age-, gender-, breed-specific IR Summary measures of disease occurrence • Prevalence (proportion) = prevalent cases/ population at a certain point of time

• Incidence proportion = new cases/ population at risk at start of study period

• Incidence rate = new cases/ total population time at risk The epidemiological toolbox • Measures of disease occurrence • Prevalence • Incidence • Cumulative incidence (or Incidence proportion) • Incidence rate • Measures of effect • Measures of strength of association • Relative risk/risk ratio • Odds ratio • (Incidence rate ratio) Measures of effect • Used to compare two groups, e.g. exposed and unexposed subjects • What is the effect of the exposure on the occurrence of the outcome? Measures of strength of association

Risk Ratio = Incidence proportionexp/ Incidence proportionunexp (a risk / another risk)

Incidence Rate Ratio = Incidence rateexp/ Incidence rateunexp).

Odds Ratio = Oddsexp/ Oddsunexp The two-by-two table

Disease + —

Exp. + a b a+b=N1

Exp. — c d c+d =N0 a+c b+d a+b+c+d=T

Relative risk or risk ratio ”Ratio of risk proportions”

Disease + —

Expo + a b a+b=N1 Expo — c Proportion d c+d =N0 a+c b+dp a+b+c+d=T

Disease Relative risk + —

Expo + a b a+b=N1

Expo — c d c+d =N0 a+c b+d a+b+c+d=T

RR = a / (a + b) c / (c + d) Notes on relative risk 1 • Exposed in numerator, unexposed in denominator. • RR=1 means no association between exposure and disease (“null value”). • RR>1 means harmful effect of exposure • RR<1 means protective effect of exposure • RR cannot be estimated in case-control studies! Example: Risk ratio (not for case control study)

Diseased Healthy Total

Exposed 13 2163 2176

Non-exposed 5 3349 3354

Total 18 5512 5530

Incidence riskexp = 13/2176 = 0.00597

Incidence riskunexp = 5/3354 = 0.00149

Risk ratio = 0.00597 / 0.00149 = 4.01 Odds Ratio

Disease + Diseased—

Expo + a Not diseased b a+b=N1

Expo — c d c+d =N0 Odds a+c p/(1-p) b+d a+b+c+d=T

Disease Odds Ratio + —

Expo + a b a+b=N1

Expo — c d c+d =N0 a+c b+d a+b+c+d=T

a/b OR = c/d Notes on odds ratios (OR) • When the disease is rare, the OR is an estimate of the risk ratio (relation between proportion and odds) • ORs can be used as an estimate of risk ratio in studies where you lack information on the total risk population (case-control studies) • OR can be calculated for all study designs Example Odds ratio

Disease Healthy Total

Exposed 13 2163 2176

Non-exposed 5 3349 3354

Total 18 5512 5530

Oddsexp = proportion/(1-proportion) , or calculate the odds:

Oddsexp= number of cases / number of healthy individuals = 13/ 2163 = 0.00601

Oddsnon-exp = number of cases /number of healthy individuals = 5/3349 = 0.00149

Odds ratio = 0.00601/0.00149 = 4.03 NB! The incidence is rare and OR ~ RR Learning goals 1. Definition of epidemiology, descriptive epidemiology Define and understand the meaning of epidemiology as a concept. Define descriptive epidemiology. Determinants of health and disease.

2. Observational studies Understand when and how we use analytic epidemiology. Describe cross-sectional study, and case-control study and their pros and cons

3. Accuracy and bias Distinguish between precision and accuracy. Know the three main types of bias, and how to control/limit them. Reliability of tests: Sensitivity Specificity Predictive values Know the meaning of internal and external validity.

4. Measures of disease and associations Understand the use of different measures of disease, and be able to calculate them. Calculate and interpret different measures of association.